Three members arrive at a meeting of a hundred-person association. Should they be able to sell its equipment or elect its next committee? A quorum rule establishes how many eligible participants must be present before the body can conduct substantive business.
A quorum is separate from the vote needed to pass a motion. Under Robert's Rules, enough people must be present before the assembly can conduct substantive business. Presence does not establish agreement, and a unanimous vote among too few people does not automatically authorize the assembly to act.
Attendance can become a veto
A club might require a third of members at its annual meeting, with the exact threshold set in its governing rules. This protects absent members from decisions made by an unusually small group. It also creates a practical obligation to make attendance possible through adequate notice, accessible times, and permitted remote participation.
An excessively high quorum can immobilize an organization with many inactive members. Organized opponents may deliberately stay away to prevent business. A low quorum solves the attendance problem by weakening the protection. Some organizations provide different procedures for reconvened meetings, but these must come from their actual rules. The threshold is a compromise between making collective action possible and requiring a meaningful portion of the group to be there when it happens.